1. Field of the Invention
The present invention relates to metalorganic chemical vapor deposition (MOCVD) formation of (Ba,Sr)(Zr,Ti)O3 perovskite crystal thin films of high breakdown strength, low leakage, low loss tangent, high permittivity, and substantial tunability with application of electric fields. Such (Ba,Sr)(Zr,Ti)O3 thin films can be used to manufacture dielectric capacitors or other related microelectronic devices of significantly improved performance useful in many applications, especially for use under elevated temperature and/or high frequency.
2. Description of the Related Art
A wide variety of semiconductor materials is used in integrated circuitry for electronic devices. Increasingly more compact integrated circuits with greater capacities are required for new devices, as well as existing device applications. This in turn necessitates the use of materials with higher specific capacitance in order to further reduce the size of individual transistors and capacitors in such integrated circuitry.
Ferroelectric materials typically have high specific capacitance due to their high permittivity ∈, which usually ranges from about 200 to about 500, making these materials correspondingly attractive as dielectric materials for capacitors. Conventional ferroelectric materials used in integrated circuit applications include ferroelectric dielectric compounds of perovskite crystalline structure, such as Pb(Zr,Ti)O3 (PZT), BaTiO3 (BT), and (Ba,Sr)TiO3 (BST).
Despite their advantages and favorable characteristics, these conventional ferroelectric materials have an associated disadvantage of relatively low breakdown strength. As a result, dielectric devices made from these materials can fail catastrophically when applied voltage rises above a specific level and causes strong short-circuit currents. In order to avoid system failure incident to such short-circuiting, applied voltage on the dielectric devices made from these materials has to be controlled carefully to keep the applied voltage below the breakdown limit. Since energy storage capacity of a capacitor positively correlates with the square of applied voltage on such capacitor, limitations on applied voltage correspondingly limit the electrical energy storage characteristics of the capacitor.
Further, conventional ferroelectric materials exhibit high current leakage under elevated voltage conditions. This in turn leads to high power dissipation as evidenced by high loss tangent xcex4 of the material. The high power dissipation rate greatly reduces the energy storage efficiency of corresponding dielectric devices fabricated from such conventional ferroelectric materials.
In contrast, many linear dielectric materials, such as SiO2 and Ta2O5, exhibit high breakdown strength. Unfortunately, the permittivity of these materials is very low, usually in the range from 3 to 20. Such low permittivity results in unsatisfactorily low specific capacitance of these dielectric materials, rendering them unsuitable for energy storage applications in integrated circuit devices.
Accordingly, there exists a compelling need for improved dielectric materials with both high specific capacitance and high breakdown strength, as well as low current leakage and low loss tangent. Dielectric materials having such combination of properties would enable manufacture of micro-capacitors or other micro-electronic devices with substantially improved energy storage and operating characteristics, relative to currently used materials.
The present invention relates in one aspect to an improved dielectric thin film comprising modified (Ba,Sr)TiO3 (BST) perovskite crystal material doped with zirconium, and to devices comprising same.
As used herein, the term xe2x80x9cthin filmxe2x80x9d refers to a film having a thickness of less than 20,000 xc3x85.
Devices utilizing dielectric Zr-doped BST perovskite crystal material within the broad scope of the present invention include, but are not limited to: electroluminescent displays (ELDs); pulse discharge capacitors; high frequency devices operated under a frequency of at least 5 MHz, more preferably in a range from 10 MHz to 40 GHz; dynamic random access memory cells (DRAMs) and ferroelectric random access memory cells (FeRAMs); microwave phase shifting and tunable varactors (variable capacitors); piezoelectric actuating elements; passive as well as active microelectromechanical system (MEMS) devices; optical devices, including both geometric and spectral- or interference-based devices, such as movable microlens arrays or movable micromirror arrays; spectral devices to alter a resonant cavity in an etalon structure to detune the reflectance of the device; micropumps and microvalves based on piezoelectric film cantilever structures, e.g., for applications such as medication dose delivery systems, or operation of hydraulic or fluidic systems in a MEMS apparatus; ultrasonic transducers, e.g., for high frequency flaw detection systems; vibration control devices; dfribillators; gate dielectrics; uncooled infrared radiation pyroelectric detectors; EEPROM and flash memory replacements; etc.
In a specific aspect, the present invention relates to a Zr-doped BST perovskite crystal material thin film formed by a MOCVD process, having high permittivity, high breakdown strength, low leakage, high-energy storage density, and high dielectric constant tunability. Such Zr-doped BST perovskite crystal material thin film in one particular aspect is characterized by at least one of the following improved dielectric properties:
a breakdown strength of at least 1.3 MV/cm, more preferably at least 1.5 MV/cm;
a leakage current of not more than 1xc3x9710xe2x88x923 A/cm2 under applied voltage of about xc2x13V or above and at temperature of about 100xc2x0 C. or above, more preferably not more than 1xc3x9710xe2x88x924 A/cm2, and most preferably not more than 1xc3x9710xe2x88x925 A/cm2, under the same applied voltage and temperature conditions; and
an energy storage density, based on volume of dielectric, of at least 15 J/cc, more preferably at least 20 J/cc, and most preferably at least 25 J/cc.
Another specific compositional aspect of the present invention relates to Zr-doped BST perovskite crystal material thin film comprising 0.5% to 50% Zr by total weight of such perovskite crystal material, preferably 2% to 15%, more preferably 4% to 14%, and most preferably 5% to 12%.
High quality thin films with low defect density tend to exhibit superior breakdown strength compared to analogous bulk material. The Zr-doped BST perovskite crystal material thin film of the present invention in a preferred embodiment has a thickness in one of the following ranges: from 150 xc3x85 to 10,000 xc3x85; from 150 xc3x85 to 5000 xc3x85; from 150 xc3x85 to 2500 xc3x85; or from 150 xc3x85 to 1000 xc3x85. More preferably such Zr-doped BST perovskite crystal material thin film is about 300 xc3x85 to about 700 xc3x85 thick, e.g., about 500 xc3x85 thick.
Growth temperature of a perovskite crystal material thin film deposited by MOCVD process has a significant impact upon crystalline structure of the thin film that is deposited, which consequently affects dielectric properties of such thin film. Carrying out the MOCVD deposition process under lower growth temperature (e.g., in a range of from about 540xc2x0 C. to about 560xc2x0 C.) tends to form films of amorphous or microcrystalline structure, which have relatively lower permittivity and energy storage capacity.
By contrast, carrying out the deposition process under higher growth temperatures (e.g., 600xc2x0 C.) significantly enhances crystal grain growth, resulting in larger crystal grains with generally longer range order and better aligned crystal lattice structure and correspondingly higher permittivity and energy storage capacity. When the growth temperature is about 660xc2x0 C., the deposited thin film becomes primarily  less than 110 greater than  oriented with the most preferred crystalline structure and dielectric properties.
Thus, a preferred embodiment of the present application relates to a Zr-doped BST perovskite crystal thin film fabricated by an MOCVD process comprising a film growth temperature (deposition temperature) in the range from about 560xc2x0 C. to about 700xc2x0 C., e.g., above about 600xc2x0 C., and more preferably from about 600xc2x0 C. to about 680xc2x0 C. A particularly preferred growth temperature for the MOCVD process of the present invention is about 660xc2x0 C.xc2x120xc2x0 C., whereby the Zr-doped BST perovskite crystal thin film formed in the process is high quality, single phase crystalline material that is substantially free of second phase formation.
Such high quality Zr-doped BST perovskite crystal thin film, containing zirconium dopant at the previously described dopant concentration levels, advantageously retains the high permittivity and high dielectric constant that are characteristic of undoped BST crystal materials. Specifically, Zr-doped BST perovskite crystal thin films of the present invention, when deposited at a growth temperature of 660xc2x0 C. with Zr content in the range from 2% to 15% by total weight of such crystal thin film, retain a high permittivity of at least 200. More preferably, the Zr-doped BST perovskite crystal thin film has a permittivity of at least 250, and most preferably, such crystal thin film has a permittivity of at least 300.
Conventional undoped BST material has high tunability, which means that its dielectric constant shifts with applied voltage or frequency and therefore becomes xe2x80x9ctunablexe2x80x9d under changing voltage or frequency conditions. This characteristic is advantageously employed in various phase shifting applications, including microwave phase shifting and tunable varactor applications. However, the conventional BST material, due to its large loss tangents, tends to dissipate electronic energy at a high rate with consequent high energy wastage. When applied voltage increases to above xc2x14V or an electric field above approximately 800 kV/cm, the loss tangent of the conventional BST material soars to as high as 0.8. Additionally, the loss tangent of the conventional BST material significantly increases at low and high frequencies.
A further aspect of the present invention therefore relates to a Zr-doped BST crystal thin film with improved electronic properties that is particularly suitable for use in microwave phase shifting and tunable varactor applications. The Zr-doped BST crystal thin film of such preferred embodiment, as modified by the B-site modifier zirconium, exhibits a reduced loss tangent of not more than 0.2 under applied voltage in the range from xe2x88x924V to +4V or an electric field in the range from about xe2x88x92800 kV/cm to +800 kV/cm, with a comparable or enhanced tunability ratio of not less than 2.5:1. The Zr-doped BST crystal thin film in such embodiment also has a reduced loss tangent of not more than 0.02 under applied frequency in the range from 10 kHz to 100 kHz.
Another aspect of the present invention relates to an MOCVD process for fabricating a dielectric thin film comprising (Ba,Sr)TiO3 perovskite crystal doped with zirconium, comprising:
preparing metal precursor liquid compositions comprising Ba, Sr, Ti, and Zr metal precursors separately or as mixtures;
flash vaporizing the liquid metal precursor compositions at a vaporization temperature in the range from about 100xc2x0 C. to about 300xc2x0 C.;
transporting the vaporized metal precursor compositions with a carrier gas into a chemical vapor deposition chamber containing a heated substrate; and
depositing the vaporized metal precursor compositions onto the heated substrate in the chemical vapor deposition chamber in the presence of an oxidizing co-reactant gas to form the dielectric thin film comprising (Ba,Sr)TiO3 perovskite crystal material doped with zirconium;
wherein the dielectric material thin film has at least one characteristic selected from the group consisting of:
(a) a breakdown strength of at least 1.3 MV/cm;
(b) a leakage current of not more than 1xc3x9710xe2x88x923 A/cm2 under applied voltage of about xc2x13V or above and at temperature of about 100xc2x0 C. or above; and
(c) an energy storage density of at least 15 J/cc.
Such MOCVD process utilizes a unique, compatible set of metalorganic precursors under specific process conditions to achieve precise compositional and microstructural control.
Such metalorganic precursors comprise Ba, Sr, Ti, and Zr metal coordination complexes including at least one ligand selected from the group consisting of xcex2-diketonates, xcex2-ketoiminates, xcex2-diiminates, C1-C8 alkyl, C2-C10 alkenyl, C2-C15 cycloalkenyl, C6-C10 aryl, C1-C8 alkoxy, and fluorinated derivatives thereof.
Metalorganic precursors including at least one ligand as above-listed are stabilized in character, meaning that the metalorganic precursors are resistant to degradation induced by ligand exchange reactions, e.g., non-degenerative ligand exchanges which adversely affect the chemical identity and suitability of the reagent compositions for MOCVD applications.
The Ba, Sr, Ti, Zr metal precursors may also be dissolved in a solvent to facilitate liquid delivery of such metal precursors. The solvent utilized for delivering the metal precursors may comprise any suitable solvent species, or combination of solvent species, with which the metal precursors are compatible, such as glymes, aliphatic hydrocarbons, aromatic hydrocarbons, ethers, esters, alkyl nitrites, alkanols, amines, and polyamines.
More preferably, the carrier solvent used in the practice of the present invention comprises tetrahydrofuran, alkyl acetate, tetraglyme, polyamines, or C3-C8 alkanols. A most preferred solvent composition useful for the purpose of delivering Ba, Sr, Ti, and Zr metal precursors comprises both butyl acetate and polyamines.
U.S. Pat. No. 5,820,664 xe2x80x9cPrecursor Compositions for Chemical Vapor Deposition, and Ligand Exchange Resistant Metal-Organic Precursor Solutions Comprising Samexe2x80x9d issued to Gardiner et al. on Oct. 13, 1998, and U.S. Pat. No. 6,110,529 xe2x80x9cMethod of Forming Metal Films on a Substrate by Chemical Vapor Depositionxe2x80x9d issued Aug. 29, 2000 to Robin Gardiner, are incorporated by reference herein. These patents contain detailed disclosure regarding metalorganic precursors and solvents for delivering such precursors, such as are useful in the broad practice of the present invention.
In a preferred embodiment of the present invention, the MOCVD process uses a strontium or barium metal precursor such as Sr bis(thd) or Ba bis(thd) coordinated with a Lewis base, e.g., a Lewis base selected from the group consisting of alkene, dierie, cycloalkene, cyclooctatetraene, alkyne, substituted alkyne (symmetrical or asymmetrical), amine, diamine, triamine, tetraamine, polyamine, ether, diglyme, triglyme, tetraglyme, tetramethyl, dialkyl sulfide, ethylenediamine, vinyltrimethylsilane, allyltrimethylsilane, and pentaamethyl diethylenetriamine. A particularly preferred Lewis base coordinating with such Sr bis(thd) or Ba bis(thd) comprises a polyamine.
Any suitable titanium or zirconium metalorganic compounds with sufficient volatility to be decomposed in the chemical vapor deposition chamber under a desirable growth temperature can be used. Specifically, the metal precursor liquid compositions of the present invention may comprise Zr(thd)4, Zr(OiPr)2(thd)2, Zr(OtBu)2(thd)2, Ti(thd)4, Ti(OiPr)2(thd)2, Ti(OtBu)2(thd)2, and combinations thereof, wherein xe2x80x9cthdxe2x80x9d denotes the ligand 2,2,6,6-tetramethyl-3,5-heptanedionato, xe2x80x9cOiPrxe2x80x9d denotes isopropoxide (xe2x80x94OCH(CH3)2), and xe2x80x9cOtBuxe2x80x9d denotes tert-butoxy (xe2x80x94OC(CH3)3).
Metalorganic compounds most preferred as metal precursors for forming (Ba,Sr)(Ti,Zr)O3 perovskite crystal thin films in the present MOCVD process include Ba(thd)2-polyamine, Sr(thd)2-polyamine, Zr(thd)4, and Ti(OiPr)2(thd)2.
A specific aspect of the present invention relates to a MOCVD process for depositing Zr-doped BST perovskite crystal thin film under a film growth temperature in the range from about 500xc2x0 C. to about 750xc2x0 C., more preferably from about 560xc2x0 C. to about 700xc2x0 C., and even more preferably from about 600xc2x0 C. to about 680xc2x0 C. The most preferred growth temperature for the MOCVD process of the present invention is about 660xc2x0 C.xc2x120xc2x0 C., whereby the product Zr-doped BST perovskite crystal thin film is a high quality, single phase crystalline film that is substantially free of second phase formation.
Such MOCVD process for depositing Zr-doped BST perovskite crystal thin film is generally carried out under a deposition pressure in the range from about 700 millitorr to about 750 millitorr, and the Zr-doped BST perovskite crystal thin film is formed on the heated substrate at a growth rate in the range of from about 10 xc3x85/min to 60 xc3x85/min.
Preferably, the carrier gas employed in the practice of the present invention for transporting the vaporized metal precursor compositions into the chemical vapor deposition chamber includes, but is not limited to, argon or other inert gas. Such carrier gas may be introduced for mixing with vaporized metal precursor compositions at a suitable flow rate, e.g., of about 200 sccm.
Oxidizing co-reactant gases useful for the broad practice of the present invention include, but are not limited to, O2, N2O, and O3. More preferably, the oxidizing co-reactant gases used in the MOCVD process of the present invention include O2 and N2O, and both gases are introduced into the chemical vapor deposition chamber at a suitable flow rate, e.g., of about 500 sccm.
In a preferred aspect, to achieve effective compositional control, the MOCVD process of the present invention delivers metal precursors into the chemical vapor deposition chamber for deposition reaction as follows:
Another aspect of the present relates to a digital MOCVD process for fabricating the (Ba, Sr) TiO3 perovskite crystal thin film doped with zirconium, including the steps of:
(i) providing a chemical vapor deposition chamber with a substrate therein;
(ii) forming over the substrate a multi-component metal precursor layer, using two or more metal precursors introduced simultaneously into the chemical vapor deposition chamber, in absence of an oxidant reactant; and
(iii) oxidizing such multi-component metal precursor layer to form the dielectric thin film comprising (Ba, Sr)TiO3 perovskite crystal material doped with zirconium, using an oxidant reactant introduced into the chemical vapor deposition chamber, in absence of the metal precursors,
in which the dielectric thin film has at least one characteristic selected from the group consisting of:
(a) a breakdown strength of at least 1.3 MV/cm;
(b) a leakage current of not more than 1xc3x9710xe2x88x923 A/cm2 under applied voltage of about xc2x13V or above and at temperature of about 100xc2x0 C. or above; and
(c) an energy storage density of at least 15 J/cc.
For more detailed disclosure of such digital MOCVD process, see U.S. Pat. No. 5,972,430 xe2x80x9cDigital Chemical Vapor Deposition (CVD) Method For Forming a Multi-Component Oxide Layerxe2x80x9d issued to DiMeo, Jr. et al. on Oct. 26, 1999, which is incorporated by reference herein.
Such digital MOCVD process may repeat steps (ii) and (iii) sequentially for a sufficient number of cycles to yield a dielectric thin film of desired thickness. Moreover, the substrate on which the dielectric thin film is deposited, may for example be a structural component of a microelectronics device or a sensor device.
Dielectric crystal thin films formed by such digital MOCVD process have improved electronic properties due to enhanced crystallinity of the films, improved uniformity and conformality upon a structured surface at comparatively lower deposition temperatures, enhanced precursor incorporation efficiency, and attenuated precursor phase reactions during deposition process.
Other aspects, features and embodiments of the present invention will become more fully apparent from the ensuing disclosure and appended claims.